Semiconductor device and method of fabricating the same

ABSTRACT

A semiconductor device comprises a single crystal substrate, a nucleus formation buffer layer formed on the single crystal substrate, and a lamination layer including a plurality of Al 1-x-y  Ga x  In y  N (0≦x≦1, 0≦y≦1, x+y≦1) layers laminated above the nucleus formation buffer layer. The nucleus formation buffer layer is formed of Al 1-s-t  Ga s  In t  N (0≦s≦1, 0≦t≦1, s+t≦1) and is formed on a surface of the substrate such that the nucleus formation buffer layer has a number of pinholes for control of polarity and formation of nuclei. A method of fabricating a semiconductor device comprises the steps of: forming, above an Al 1-x-y  Ga x  In y  N (0≦x≦1, 0≦y≦1, x+y≦1) semiconductor layer doped with a p-type dopant, a cap layer for preventing evaporation of a constituent element of the semiconductor layer, the cap layer being formed of one of AlN in which a p-type dopant is added and A1 2  O 3 , subjecting the semiconductor layer to heat treatment, and removing at least a part of the cap layer.

This is a Continuation of application Ser. No. 08/400,865, filed on Mar.8, 1995, now U.S. Pat. No. 5,656,832.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device using anAlGaInN-based material and a method of fabricating the same, and relatesto an improvement of a buffer layer provided between a substrate and asemiconductor device layer structure, a semiconductor device having ap-type semiconductor layer with reduced resistance and a method offabricating the same.

2. Description of the Related Art

One of nitrogen-containing III-V group compound semiconductors, i.e.GaN, which has a large band gap of 3.4 eV and is of direct transitiontype, has been regarded as a prospective material of a short-wavelengthlight emitting device. Since there is no substrate with good latticematching with this type of material, the material is often grown on asapphire substrate. However, since the possibility of non-matchingbetween sapphire and GaN is high, i.e. about 15%, the material tends togrow in an insular shape. Furthermore, if the thickness of a GaN layeris increased to enhance the quality thereof, a difference in thermalexpansion between a sapphire substrate and GaN or Al_(1-x-y) Ga_(x)In_(y) N (where 0≦x≦1, 0≦y≦1, x+y≦1, hereinafter referred to asAlGaInN-based material) and lattice non-matching result in an increasein dislocation or cracks at the time of growing and/or cooling.Consequently, it is difficult to grow a high-quality film.

On the other hand, in order to reduce the influence of latticenon-matching, there is a known method in which a very thin film ofamorphous or polycrystal AlN or GaN is formed as a buffer layer on asapphire substrate by low-temperature growth. In this case, it isconsidered that the amorphous or polycrystal buffer layer reducesthermal distortion, small crystals contained in the buffer layer becomeorientated seeds at high temperature of 1000° C., and the crystalquality of the GaN layer is enhanced.

In the case of adopting this method, the crystal quality represented by,e.g. a full width at half maximum of x-ray diffraction depends greatlyon the growth conditions of the buffer layer. Specifically, when thethickness of the buffer layer is great, the orientation of the seeds,which become the nuclei of the film formation, are disturbed and thecrystal quality deteriorates. On the other hand, although the full widthat half maximum decreases as the thickness of the buffer layerdecreases, the function of the buffer layer is completely lost by anexcessively small thickness of the buffer layer and the surfacecondition of the crystal deteriorates suddenly. In other words, thegrowth conditions of the buffer layer are strictly limited and thecrystal quality is not satisfactory.

As has been stated above, in the prior art, it is difficult tocrystal-grow a high-quality AlGaInN-based think film on the sapphiresubstrate. Moreover, even if the amorphous or polycrystal buffer layeris used, the growth conditions of the buffer layer are strictly limitedand the crystal quality of the AlGaInN-based thin film formed on thebuffer layer is not satisfactory. Therefore, it is difficult tofabricate a semiconductor light emitting device with high luminance andshort wavelength by using the AlGaInN-based material.

When this type of semiconductor device is used as a semiconductor laser,etc., it is necessary to provide means for forming a low-resistancep-type layer. In the prior art, it is difficult to grow a low-resistancep-type layer with GaN. Recently, however, the resistance of the GaNlayer can be decreased by radiating an electron beam or by heating in anitrogen gas atmosphere the GaN layer to which Mg is added. It appearsthat the resistance can be decreased by virtue of dissociation ofhydrogen from the crystal.

The inventors of the present invention, however, have discovered thatthe electrical activation ratio of Mg in the GaN layer is still low, andit is necessary to add Mg at a very high concentration of about 10¹⁹cm⁻³ to about 10²⁰ cm⁻³ in order to obtain a low-resistive p-type layerof 1 Ωcm or less which is necessary to fabricate a high-performancedevice such as a semiconductor laser. The addition of Mg at highconcentration results in an increase in crystal defects anddeterioration in surface flatness. Thus, it is not possible to achieve ahigh-performance, short-wavelength semiconductor laser, etc.

On the other hand, Jap. Pat. Appln. KOKAI Publication No. 5-183189proposes a method of a low-resistance p-type GaN layer by forming an AlNlayer as a cap layer on a GaN layer to which Mg is added, and thenannealing the resultant. In this method, however, the electricalactivation of Mg is not fully achieved.

As has been described above, in the prior art, when a semiconductorlayer of an AlGaInN-based compound (a nitride-series Group III-Vcompound semiconductor layer) is grown, it is necessary to add Mg(acceptor impurity) excessively as a dopant in order to low-resistancep-type layer. As a result, crystal defects increase and surface flatnessdeteriorates.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a semiconductor deviceand a method of fabricating the same, wherein a high-quality AlGaInNsemiconductor layer can be formed with high reproducibility on asubstrate having no lattice match and, for example, a high-luminance,short-wavelength semiconductor light emitting device can be realized.

Another object of the invention is to provide a method of fabricating asemiconductor device, wherein the electrical activation ratio ofacceptor impurity in the semiconductor layer is increased, thereby toform a low-resistance p-type layer, and a high-performance,short-wavelength semiconductor laser, etc. can be realized.

In order to achieve the above object, the present invention has theconstructions stated below.

A semiconductor device according to a first aspect of the inventioncomprises:

a single crystal substrate;

a nucleus formation buffer layer formed on the single crystal substrate;and

a lamination layer including of a plurality of Al_(1-x-y) Ga_(x) In_(y)N (0≦x≦1, 0≦y≦1, x+y≦1) layers laminated above the nucleus formationbuffer layer,

wherein the nucleus formation buffer layer is formed of Al_(1-s-t)Ga_(s) In_(t) N (0≦s≦1, 0≦t≦1, s+t≦1) and is formed on a surface of thesubstrate such that the nucleus formation buffer layer has a number ofpinholes for control of polarity and formation of nuclei.

The following are desirable modes for working the invention of the firstaspect:

(1) The nucleus formation buffer layer is loosely formed on the surfaceof the substrate to a very small thickness such that the buffer layerhas a number of pinholes, and an average thickness of the buffer layeris 3-10 nm.

(2) The nucleus formation buffer layer is composed of AlN.

(3) A thermal distortion reducing buffer layer of InN or GaInN isprovided on the nucleus formation buffer layer.

(4) A cap layer for preventing evaporation of In of the buffer layer isformed on the thermal distortion reducing buffer layer.

(5) The growth temperature of the buffer layer for forming nuclei andreducing thermal distortion is 350 to 800° C., and preferably 500 to700° C.

(6) A temperature raising process after the formation of the bufferlayer and before the start of growth of a semiconductor device layer isperformed in an ammonia-free atmosphere.

(7) The growth of the semiconductor device layer is performed at 70 Torror below, and preferably 1 to 40 Torr.

(8) The single crystal substrate is a sapphire substrate, preferably asapphire substrate with a C face.

(9) The semiconductor layer formed on the buffer layer constitutes alight emitting diode with a double-hetero structure having an activelayer sandwiched by p-type and n-type cladding layers.

A method of fabricating a semiconductor device, according to a secondaspect of the invention, comprises the step of forming a p-type layer ofAl_(1-x-y) Ga_(x) In_(y) N (0≦x≦1, 0≦y≦1, x+y≦1) above a single crystalsubstrate,

wherein after the p-type layer is formed, the resultant structure iscooled in a higher temperature range between a growth temperaturethereof and 850° C. to 700° C. in ammonia containing atmosphere to curbdissociation of nitrogen atom and is cooled in a lower temperature rangeunder the higher temperature range in an ammonia-free gas to curb mixingof hydrogen atom during cooling.

A method of fabricating a semiconductor device, according to a thirdaspect of the invention, comprises the steps of:

forming a p-type layer of Al_(1-x-y) Ga_(x) In_(y) N (0≦x≦1, 0≦y≦1,x+y≦1) above a single crystal substrate;

subjecting the p-type layer to heat treatment in an active nitride,thereby increasing an activation ratio of a p-type dopant of the p-typelayer.

A method of fabricating a semiconductor device, according to a fourthaspect of the invention, comprises the steps of:

forming, above an Al_(1-x-y) Ga_(x) In_(y) N (0≦x≦1, 0≦y≦1, x+y≦1)semiconductor layer doped with a p-type dopant, a cap layer forpreventing evaporation of a constituent element of the semiconductorlayer, the cap layer being formed of one of AlN in which a p-type dopantis added and A1₂ O₃ ;

subjecting the semiconductor layer to heat treatment; and

removing at least a part of the cap layer.

The following are desirable modes for working the invention:

(1) The cap layer is formed of AlN in which a p-type dopant (acceptor)is added, and the concentration of doped acceptor in AlN is 10¹⁷ to 10²⁰cm⁻³.

(2) The heat treatment temperature is 700° C. to 1200° C.

(3) The semiconductor layer is formed of GaN or AlGaN, and the acceptorimpurity is Mg.

(4) A method of fabricating a semiconductor device comprises the stepsof:

forming a buffer layer of Al_(1-x-y) Ga_(x) In_(y) N (0≦x≦1, 0≦y≦1,x+y≦1) on a single crystal substrate; and

successively forming, on the buffer layer, an n-type cladding layer, anactive layer, a p-type cladding layer and a p-type contact layer whichare all formed of Al_(1-s-t) Ga_(s) In_(t) N (0≦s≦1, 0≦t≦1, s+t≦1).

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention, and together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIG. 1 is a characteristic graph showing the relationship between thethickness of an AlN buffer layer and the full width at half maximum atX-ray diffraction of a GaN layer;

FIG. 2 is a cross-sectional view showing the device structure of a lightemitting diode according to a first embodiment of the invention;

FIGS. 3A to 3E are cross-sectional views of light-emitting diodesaccording to modifications of the first embodiment;

FIG. 4 is a cross-sectional view showing schematically the structure ofa growing apparatus used in fabricating the device of the firstembodiment;

FIG. 5 is a cross-sectional view showing schematically the structure ofan annealing apparatus used in the first embodiment;

FIG. 6 is a cross-sectional view showing the device structure of alight-emitting diode according to a second embodiment of the invention;

FIGS. 7 and 8 are cross-sectional views of light-emitting diodesaccording to modifications of the second embodiment;

FIG. 9 is a characteristic graph showing the relationship between theflow rate of ammonia and the growth rate at the time of growing GaN;

FIG. 10 is a characteristic graph showing the relationship between thetemperature for thermal treatment of GaN and the hydrogen concentrationin GaN;

FIG. 11 is a characteristic graph showing the relationship between thetemperature for thermal treatment and resistance value in GaN to whichMg is added;

FIG. 12 is a characteristic graph showing the relationship between theMg concentration and the resistance of film when a GaN layer wassubjected to heat treatment in an Ar atmosphere;

FIG. 13 is a cross-sectional view showing schematically the structure ofa semiconductor laser according to a third embodiment of the invention;and

FIG. 14 is a cross-sectional view of a semiconductor laser in the statein which an AlN cap layer is formed for a process of hydrogen removal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to describing embodiments of the present invention, a descriptionwill first be given of a buffer layer between a substrate and asemiconductor device lamination structure.

The inventors of the present invention discovered through researchesthat nucleus formation for controlling polarity of growth surface issubstantially important as a function of a buffer layer, in addition toreduction of lattice non-matching, which has been conventionallythought. Specifically, if a GaN layer is directly grown on a sapphiresubstrate without providing a buffer layer, nitrogen material reactswith substrate crystals. Since sapphire has a nonpolar crystalstructure, the polarity of a produced nitride is disturbed.

On the other hand, when a buffer layer is grown at low temperaturesbelow 700° C., material molecules functioning as an N (nitrogen) supplysource of Group V element or a decomposed substance thereof stay on thesubstrate surface effectively, and an N atom surface is first formed.Thereby, the growth surface is controlled to surface A on which GroupIII atoms appear. Accordingly, when ammonia with low decomposition ratiois used as N material, the formation of a Group V atomic surface(surface B), which may become unstable due to deficiency of N material,can be curbed. This is a principal reason why the quality of crystals inthe buffer layer formed by low-temperature growth can be improved.

Therefore, the nucleus formation for polarity control of the growthsurface is important as a function of the buffer layer. It is notnecessary that the nuclei having this function be present as a completefilm. Rather, it is desirable that the nuclei be formed as a film withmany pinholes. This enhances the crystal quality, independent of growthconditions, thickness, etc. of the buffer layer. Normally, at thesubstrate temperature above 800° C., nuclei of GaN do not easily form onthe surface of the sapphire substrate. If a low-temperature growthbuffer layer is provided, GaN grows from nuclei preformed at lowtemperatures in a horizontal direction along the surface of thesubstrate. It is considered that there is substantially no crystaldefects due to lattice non-matching in a region grown from the nucleihorizontally.

In the prior art, in the case where the buffer layer is thin, suddendegradation in crystal quality is considered to occur due to the factthat the substrate reacts directly with nitrogen material and a portionwith disturbed polarity forms. Specifically, in the case where a GaNlayer for forming a semiconductor device is grown on a sapphiresubstrate with, e.g. an AlN buffer layer interposed, a Group IIImaterial, e.g. TMA (tri-methyl-aluminum), and a Group V material, e.g.NH₃, are supplied to form a buffer layer. Then, the supply of the GroupIII material is stopped, and the resultant is heated up to apredetermined temperature. Subsequently, another Group III material,e.g. TMG (tri-methyl-gallium), is supplied to start the growth of a GaNlayer. At this time, since the Group V material is kept being supplied,the substrate reacts directly with ammonia or the Group V material inthe temperature-increasing process if the buffer layer is thin.

By contrast, if the temperature-increasing process is performed in anatmosphere containing a small amount of nitrogen material alone forpreventing dissociation of nitrogen atoms, and containing no ammonia,for example in a hydrogen atmosphere, the nuclei can be formed withoutnitriding the surface of the substrate. In this case, however, gaseshaving sharply different thermal qualities, such as hydrogen andammonia, are interchanged after the temperature is raised and thethermal quality of the atmospheric gas is varied. Consequently, thesurface temperature of the substrate varies. The inventors discoveredthe fact that it is important that the growth be performed under areduced pressure of 70 Torr or less, preferably 1-40 Torr, at which thethermal conductivity of gases decreases steeply.

FIG. 1 shows the relationship between the thickness of an AlN bufferlayer heated in hydrogen and the full width at half maximum at X-raydiffraction of a GaN layer grown on the AlN buffer layer. When thethickness of the buffer layer is less than 10 nm, i.e. in the range of3-8 nm, an epitaxial layer of much higher quality than in the prior artis obtained. In this case, the buffer layer is not a complete film, buta film with many pinholes and loosely formed small AlN crystals. Thefact that a high-quality epitaxial layer can be obtained with thethickness of the buffer layer being less than 10 nm means that theconditions for growth of the buffer layer are relaxed, and thiscontributes to higher productivity.

When a buffer layer with many pinholes is formed, layer portions growingon those areas of the substrate surface, which are exposed by pinholes,grow from small nuclei. Thus, lateral growth is facilitated and a layerwith small defect can be grown. When it is needed to more facilitatelateral crystal growth, the best result will be obtained by using asubstrate having a sapphire C face. If a substrate with a variance inthe face orientation or with surface defects is used, it is effective touse a substrate with an inclination of 0.5° to 10° (preferably 1° to 5°)from C face to A face. By using such an inclined-face substrate, a filmwith higher quality can be obtained.

The interval of AlN small crystals functioning as nuclei is determinedby the temperature for growth, and this interval increases as thetemperature rises. For smooth lateral growth, high temperatures at whichthe interval of nuclei increases are desirable. However, inhigh-temperature growth the polarity of nuclei is disturbed and thetemperature for growth of the buffer layer is limited. Good results wereobtained in the range of 350° C. to 800° C., preferably in the range of500° C. to 700° C.

In this method, too, if the thickness of a GaN layer is increased toenhance the quality thereof, the growth temperature of the GaN is sethigh at about 1000° C. Thus, a dislocation will increase at the time ofcooling owing to a difference in thermal expansion between the sapphiresubstrate and GaN (or AlGaInN) or cracks will occur. Accordingly, inorder to reduce thermal distortion, it is necessary to thicken thebuffer layer, lower the growth temperature, and reduce the distortiondue to the difference in temperature. However, if the first buffer layerfor forming nuclei is thickened, the orientation of the seeds whichbecome growth nuclei is disturbed, resulting in degradation in crystalquality. Thus, in the present invention, a second buffer layer forreducing thermal distortion is laminated on the first buffer layer forforming the growth nuclei.

It is not necessary that the buffer layer for reducing thermaldistortion be amorphous or polycrystalline. Accordingly, a materialcontaining In (indium) as a constituent element, which has beenconsidered to tend to become a single crystal because of its lowcrystallization temperature, may be used as a second buffer layer.Specifically, a bonding force acting between In and N is weak and In hasflexibility in relation to AlN. Thus, a buffer layer containing In as aconstituent element can effectively reduce distortion. It should benoted, however, that the second buffer layer may be formed of, otherthan the material containing In as a constituent element, a materialhaving a wider band gap than the first buffer layer, which has generallygood flexibility. In this case, it is more effective to use a bufferlayer of substantially a single crystal which permits an increase infilm thickness.

The effective thickness of the second buffer layer for reducing thermaldistortion is in a range of 50 nm to 1000 nm. When the In composition is10 atom % to 90 atom %, the layer grows most easily. Although In has ahigh mobility and can be formed in a wide temperature range of 300° C.to 1100° C., nuclei are difficult to form. Thus, in order to grow abuffer layer of a material including a great quantity of In as aconstituent element, it is desirable to grow in advance a layer having asmall In composition.

As has been described above, in order to form small growth nuclei, it iseffective that the first buffer layer for forming the nuclei is made ofa material having a wide band gap and, e.g. a great Al composition. Itis also effective that the second buffer layer for reducing thermaldistortion is formed of a material having a narrow band gap and, e.g. agreat In composition. When a device structure is formed of anAlGaInN-based material on the second buffer layer for reducing thermaldistortion, it is desirable that a cap layer not containing In, such asGaN, AlN or AlGaN, be preformed in a substrate temperature range of 500°C. to 800° C. at which dissociation of In is not quick, therebypreventing dissociation of In of the second buffer layer. It isdesirable that the thickness of the cap layer be in a range of 50 nm to1000 nm.

In the present invention, the term "buffer layer" refers to a layerhaving a film shape or many pinholes for the purpose of formation ofnuclei, control of polarity, reduction in thermal distortion, etc.

According to the present invention, a buffer layer of AlN, etc. havingmany pinholes is formed on a single crystal substrate of sapphire, etc.,so that AlN small crystals are loosely formed on the substrate. Thesesmall crystals become nuclei for lateral epitaxial growth of asemiconductor layer. After the buffer layer is formed, the resultant isheated in, e.g. a hydrogen atmosphere containing no ammonia until thegrowth of a plurality of semiconductor layers is started to form asemiconductor device. Thereby, a reaction between the substrate surfaceand nitrogen can be prevented and the disturbance in polarity of thesubstrate surface can be prevented. Therefore, the crystal quality andreproducibility of the plural semiconductor layers formed on the bufferlayer can be enhanced. As a result, an AlGaInN layer with less defectscan be grown, and a high-luminance short-wavelength light emittingdevice can be realized.

Besides, by forming a second buffer layer of InN, GaInN, etc. on a firstbuffer layer of AlN, etc., the second buffer layer can function as athermal distortion reducing layer and the crystal quality of a pluralityof semiconductor layers formed on the buffer layer can be enhancedeffectively.

Embodiments of the present invention will now be described withreference to the accompanying drawings.

(Embodiment 1)

FIG. 2 is a cross-sectional view showing a device structure of a bluelight emitting diode according to a first embodiment of the invention.Specifically, an AlN first buffer layer 11 (9 nm) for forming growthnuclei and controlling polarity is formed on a C face of a sapphiresubstrate (single crystal substrate) 10 at 580° C. An InN second bufferlayer 12 (0.5 μm) for reducing thermal distortion is formed on theresultant at 500° C. A GaN gap layer 13 (0.1 μm) for preventingevaporation on of In is formed on the resultant.

After the layers 11-13 are formed, the resultant is heated up to 1050°C. in an ammonia-free hydrogen atmosphere, and the following layers aresuccessively formed: an Ga₀.7 In₀.3 N defect reducing layer 14 (3.0 μm)for reducing a lattice defect; an Si-doped n-type Al₀.2 Ga₀.45 In₀.35 Ncladding layer 15 (1.0 μm) functioning as a light emitter; a Ga₀.7 In₀.3N active layer 16 (0.5 μm); an Mg-doped p-type Al₀.2 Ga₀.45 In₀.35 Ncladding layer 17 (1.0 μm); and an Mg-doped p-type GaN contact layer 18(0.5 μm).

An Au/Cr/Pd layer is formed as a p-side electrode 21 on the contactlayer 18. An Au/AuGe layer is formed as an n-side electrode 22 on thedefect reducing layer 14.

In this structure, the AlN first buffer layer 11 is loosely formed onthe substrate 10 and has many pinholes. The AlN first buffer layer 11becomes effective nuclei for growth of an AlGaInN-based semiconductorlayer for fabricating a device in a subsequent process. Further, the InNsecond buffer layer 12 functions as a thermal distortion reducing layerand can prevent dislocation and cracks due to a difference in thermalexpansion between the AlGaInN-based semiconductor layer and thesubstrate 10. Specifically, a high-quality AlGaInN-based semiconductorlayer can be formed by virtue of the functions of the two buffer layers11 and 12, and a high-luminance short-wavelength light emitting diodecan be realized.

FIGS. 3A and 3B show structures wherein the band gap of the active layer16 is varied to alter the wavelength of emitted light. FIG. 3A shows anexample of a green light emitting diode wherein the composition of adefect reducing layer 14' is Ga₀.5 In₀.5 N, the composition of claddinglayers 15' and 17' is Al₀.2 Ga₀.25 In₀.55 N, and the composition of anactive layer 16' is Ga₀.5 In₀.5 N. FIG. 3B shows an example of a redlight emitting diode wherein the composition of a defect reducing layer14" is Ga₀.3 In₀.7 N, the composition of cladding layers 15" and 17" isAl₀.2 Ga₀.05 In₀.75 N, and the composition of an active layer 16" isGa₀.3 In₀.7 N.

FIG. 3C shows a structure wherein a Ga₀.5 In₀.5 N mixed crystal is usedas a second buffer layer 32 for reducing thermal distortion and AlGaN isused as a cap layer 33. In this structure, the first buffer layer 11 forforming nuclei may be omitted, as shown in FIG. 3D. Another mixedcrystal of Al₀.5 In₀.5 N, etc. may be similarly used as thermaldistortion reducing buffer layer 32. In the case where a mixed crystalis used as second buffer layer 32, the cap layer 33 for preventingevaporation of In may be omitted, as shown in FIG. 3E, since evaporationof In is slow.

FIG. 4 shows schematically the structure of a growing apparatus used infabricating the device of this embodiment. Reference numeral 41 is areaction tube of quartz into which a material mixture gas is introducedfrom a gas introducing port 42. The gas within the reaction tube 41 isexhausted from a gas exhaust port 43.

A susceptor of carbon 44 is disposed within the reaction tube 41, and asubstrate 47 is mounted on the susceptor 44. The susceptor 44 is heatedby induction using a high-frequency coil 45. The temperature of thesubstrate 47 is measured by a thermocouple 46 and controlled by acontrol apparatus (not shown).

A description will now be given of a method of fabricating a lightemitting diode with use of the growing apparatus (shown in FIG. 4). Atfirst, the substrate 47 (sapphire substrate 10 in FIG. 2) is mounted onthe susceptor 44. High-purity hydrogen is introduced from the gasintroducing port 42 at a rate of 1 l/min., and the gas within thereaction tube 41 is replaced. Then, the gas exhaust port 43 is connectedto a rotary pump and the pressure within the reaction tube 41 is reducedto 20 to 70 Torr.

Subsequently, the substrate 47 is heated within hydrogen at 1100° C. andthe surface of the substrate 47 is cleaned. Following this, thetemperature of the substrate is lowered to 450 to 900° C., and H₂ gas ischanged to NH₃ gas, N₂ H₄ gas, or an N-containing organic compound, e.g.(CH₃)₂ N₂ H₂. In addition, an organic metal compound according to alayer to be grown is introduced, thereby forming the layer. When the AlNbuffer layer 11 is formed, for example, an organic metal Al compound,e.g. Al(CH₃)₃ or Al(C₂ H₅)₃ is introduced. When the GaN second bufferlayer 12, active layer 14 and contact layer 16 are formed, an organic Gacompound, e.g. Ga(CH₃)₃ or Ga(C₂ H₅)₃ is introduced to form them. Whenthe AlGaN cladding layers 13 and 15 are formed, both of an organic metalAl compound and an organic metal Ga compound are introduced to formthem. In order to narrow the band gap of the GaN active layer 14, In(indium) may be added. In this case, In is added by introducing anorganic metal In compound, e.g. In(CH₃)₃ or In(C₂ H₅)₃.

When doping is carried out, a doping material is introducedsimultaneously. An n-type doping material for forming the cladding layer13 is an Si hydride such as SiH₄ or an organic metal Si compound such asSi(CH₃)₄. A p-type doping material for forming the cladding layer 15 andcontact layer 16 is an organic metal Mg compound such as (C₅ H₅)₂ Mg oran organic metal Zn compound such as Zn(CH₃)₂, etc. In order to improvethe ratio of incorporated In, a layer containing In is grown in anatmosphere containing no nitrogen, Ar, etc., and (CH₃)₂ N₂ H₂ having ahigher decomposition ratio than ammonia is used as material.

In order to increase the ratio of activation of p-type dopant, it isnecessary to prevent mixing of hydrogen into a crystal. For thispurpose, in the higher temperature range between the growth temperatureand 850° C. to 700° C., the substrate is cooled in ammonia in order toprevent dissociation of nitrogen. In the lower temperature range under850° C. to 700° C., the substrate is cooled with a gas containing no NH₃in order to prevent mixture of hydrogen from NH₃ or a carrier gas in thecooling process. Furthermore, when it is necessary to increase the ratioof activation of p-type dopant, thermal treatment is effected in anitrogen radical produced by an RF plasma. This is for the reason thatheat treatment can be performed at high temperatures of 900° C. to 1200°C., owing to the fact that dissociation of nitrogen atoms from thecrystal can be prevented perfectly, and crystalline defects such asnitrogen cavities can be eliminated.

Specifically, films are grown such that NH₃ is introduced as material ata rate of 1×10⁻³ mol/min., Ga(CH₃)₃ is introduced at a rate of 11×10⁻⁵mol/min., and Al(CH₃)₃ is introduced at a rate of 1×10⁻⁶ mol/min. Thetemperature of the substrate is 1050° C., the pressure is 38 Torr, thetotal flow amount of material gas is 1 l/min., the n-type dopant is Si,and the p-type dopant is Mg. The materials are Si(CH₃)₄ and (C₅ H₅)₂ Mg,respectively.

The thus obtained semiconductor substrate was evaluated by X-raydiffraction. It was found that the crystalline defect reducedremarkably, and realization of high-luminance, short-wavelength lightemitting devices could be expected. On the other hand, it is possible tocurb release of N in annealing and reduce the resistance of p-typelayers 17 and 18, by annealing the semiconductor substrate in a nitrogenradical at 400° C. to 1200° C. (preferably 700° C. to 1000° C.). FIG. 5shows schematically an annealing apparatus. Reference numeral 91 denotesa reaction tube, numeral 92 a wafer, 93 a susceptor with an additionalfunction of a heater, 94 a high-frequency coil for activating a gas, and95 a high-frequency power supply. Specifically, the semiconductorsubstrate was annealed within the shown apparatus at 1000° C. for 30minutes.

It is also effective that annealing is performed in anitrogen-containing compound (an active nitride) which releases noactive hydrogen. Specifically, if annealing is carried out in an organiccompound having an azide group, e.g. ethyl azide, dissociation of N canbe prevented in the annealing and H is not incorporated. Thus, theresistance of the p-type layer can be further reduced.

(Embodiment 2)

FIG. 6 is a cross-sectional view showing the device structure of a lightemitting diode according to a second embodiment of the presentinvention. In this embodiment, a contact layer is provided not only onthe p-side but also on the n-side, thereby further enhancing theefficiency.

An AlN first buffer layer 51 (9 nm) for forming a growth nucleus andcontrolling polarity is formed on the C face of a sapphire substrate 50at 350° C., and then a Ga₀.5 In₀.5 N second buffer layer 52 (0.5 μm) forreducing thermal distortion is formed at 550° C. A GaN cap layer 53 (0.5μm) for preventing In evaporation is formed on the second buffer layer52 at 650° C.

After the layers 51 to 53 are formed, the resultant is heated up to1050° C. in an ammonia-free hydrogen atmosphere and the following layersare successively formed: an Se or S doped n-type GaN contact layer 54(2.0 μm); an Se or S doped GaInN (from GaN to Ga₀.7 In₀.3 N) compositiongrading layer 55 (1.0 μm) for reducing lattice mismatch; an Se or Sdoped Ga₀.7 In₀.3 N defect-reducing layer 56 (4.0 μm) for reducingcrystalline defect; an Se or S doped (1×10¹⁸ cm⁻³ n-type Al₀.1 Ga₀.55In₀.35 N cladding layer 57 (1.0 μm) functioning as a light emitter; aGa₀.7 In₀.3 N active layer 58 (0.5 μm); an Mg or Zn doped (1×10¹⁸ cm⁻³)p-type Al₀.1 Ga₀.55 In₀.35 N cladding layer 59 (1.0 μm); and an Mg or Zndoped (5×10¹⁸ cm⁻³) p-type GaN contact layer 60 (0.5 μm).

Subsequently, Pd: 500 nm, Cr: 100 nm and Au: 500 nm are formed on thecontact layer 60, and AuGe: 100 nm, and Au: 500 nm are formed on thecontact layer 54. The resultant is subjected to heat treatment at 400°C. to 800° C. in an inert gas or N₂, thus forming ohmic electrodes(p-side electrode 61 and n-side electrode 62).

With this structure, too, a high-quality AlGaInN-based semiconductorlayer can be formed by virtue of the functions of the AlN first bufferlayer 51 and GaInN second buffer layer 52, and the same advantage aswith the first embodiment can be obtained. In addition, in the presentembodiment, since a lattice mismatch of 0.3% is provided between theactive layer 58 and cladding layers 57 and 59, the light emissionwavelength is increased and the degree of absorption can be reduced.

In the present embodiment, the composition grading layer 55 is providedfor reducing lattice mismatch. However, grading is not necessarilyrequired. The thermal distortion reducing layer is not limited to GaInN,and GaN can also be used, as shown in FIG. 7. In this case, an AlN firstbuffer layer 51 (9 nm) for forming growth nuclei and controllingpolarity is grown on a C face of a sapphire substrate 50 at 350° C. AGaN second buffer layer 72 (0.5 μm) for reducing thermal distortion isgrown on the first buffer layer 51 at 550° C. Subsequently, layers 54 to60 are formed similarly with the structure as shown in FIG. 6.

No buffer layer for reducing thermal distortion may be provided, asshown in FIG. 8. An AlN first buffer layer 51 (average thickness: 5 nm)for forming growth nuclei and controlling polarity is grown on asubstrate 50 which is set off by 5° from a C face to an A face ofsapphire. Subsequently, layers 54-60 are formed similarly with thestructure as shown in FIG. 6.

In order to form nuclei, a layer having many pinholes formed loosely isdesirable, since it facilitates growth in a lateral direction and as aresult a high-quality layer can be formed. When growth is effected on anA face of sapphire, stripes may often appear on the grown surface.However, by using a buffer layer with pin holes, mirror-surface growthcan be realized. In addition, GaN may be used as buffer layer forforming the nuclei. In this case, dissociation of nitrogen can beprevented by introducing a very small amount of ammonia to the lastlimit at which GaN grows.

FIG. 9 is a graph showing the relationship between the flow rate ofammonia and the speed of growth at the time of growing GaN. Even if theflow rate of ammonia is reduced to 1/200 (5 ml/min.) of the total flowrate, GaN grows. When the flow rate of ammonia is about 1/50 (20ml/min.), the film thickness becomes the greatest. When ammonia of about1/50 to 1/200 of the total flow rate is introduced, the dissociation ofnitrogen is most reduced. Under such conditions, GaN can be used as abuffer layer material for forming the nuclei.

The present invention is not limited to the above embodiments. Thedevice structures are not limited to those shown in the embodiments andcan be modified where necessary. In brief, the present invention isapplicable to the case where a semiconductor layer of AlGaInN materialis formed on a single crystal substrate to form a light emittingelement, etc. Besides, the substrate is not necessarily limited to asapphire substrate, and a single crystal of an SiC or other material maybe used. The present invention is not necessarily limited to a lightemitting device, can be applicable to, e.g. a high-temperature operablesemiconductor device. Other modifications may be made without departingfrom the spirit of the present invention.

As has been described above in detail, according to the presentinvention, the crystalline quality and reproducibility of asemiconductor layer for forming a device made of an AlGaInN-basedmaterial can be improved. As a result, an AlGaInN-based semiconductorlayer of small defect can be grown, and a high-luminance,short-wavelength light emitting device, etc. can be fabricated.

Now consideration will be given of Mg used as a p-type dopant (acceptorimpurity) of a nitride-based Group III-V compound semiconductor. If theactivation ratio of Mg can be increased, it becomes possible to reducethe amount of added Mg and obtain a p-type layer with good surfaceflatness, small defect and low resistance. The inventors of the presentinvention examined the hydrogen concentration in a crystal, which isconsidered to be closely related to a decrease in activation ratio ofMg. As a result, it was found that hydrogen of a concentrationsubstantially equal to a high Mg concentration was mixed in a dopedlayer, as compared to an undoped layer, and that the amount of mixedhydrogen could be reduced by heat treatment.

FIG. 10 is a graph showing the relationship between the temperature forheat treatment and hydrogen concentration when the heat treatment wasperformed at various temperatures. A sample is Mg-doped GaN. Thehydrogen concentration can be decreased by performing heat treatment attemperatures of 700° C. or above. In the case of a device requiringexact control of carrier concentration as in a semiconductor laser, itis necessary to limit the hydrogen concentration to 10% or less of thesaturation concentration. In this case, treatment needs to be performedat 1000° C. or above. However, if treatment is performed at 800° C. orso, the degree of evaporation of a constituent element such as nitrogenincreases steeply and the crystalline surface deteriorates considerably.

In order to prevent the deterioration of the surface due tohigh-temperature treatment, it is important to prevent evaporation of aconstituent element such as nitrogen. According to the study by theinventors, it was found that evaporation of the constituent element canbe curbed in a high temperature region where hydrogen can be removed, bychoosing a proper method. Specifically, the surface is coated withanother material (cap layer) and then subjected to heat treatment. It isimportant that the cap layer has heat resistance, does not include anelement (e.g. donor type impurity such as Si or Se) adversely affectingthe formation of p-type nitride layer due to diffusion, has goodhydrogen permeability, has a lattice constant close to that of a nitrideand no thermal distortion, and is a dense film non-permeable to nitrogenor Ga. In addition, since a cap layer is removed after completion ofheat treatment, it is desirable that the cap layer be formed of amaterial which can be etched selectively in relation to an underlyingnitride layer.

Of these conditions, the most important condition is that the cap layerincludes no impurities forming a donor type or deep level which is afactor of an increase in resistance. Thus, a Group III-V compoundsemiconductor is suitable. Of the III-V compound semiconductors, thosehaving lattice constant close to the lattice constant of a nitride are anitride itself and BP (boron phosphide). However, BP is chemicallystable and difficult to etch. In addition, GaN is difficult toselectively etch, and InN lacks heat resistance.

On the other hand, AlN has good heat resistance and can be selectivelyetched with hydrochloric acid, but these compounds have a problemrelating to hydrogen permeability. However, according to the inventors'study, it was thought that the hydrogen permeability of the Group III-Vnitride was remarkably increased by addition of an acceptor. Forexample, in the case of AlN, adequate hydrogen permeability can beobtained by adding Mg in an amount of 10¹⁷ to 10²⁰ cm⁻³. In particular,by doping the cap layer with the same impurity as the acceptor impurityof the cladding layer, it is possible to prevent an increase in contactresistance due to a decrease in carrier concentration of the surfacebeing subjected to heat treatment. Accordingly, by using theacceptor-doped AlN as cap layer, hydrogen can be removed while curbingevaporation of the constituent element such as nitrogen.

A1₂ O₃ is a substrate material conventionally used for forming a nitridelayer as well, though its lattice constant is different from that of anitride, and is free from the problem of involving a great thermaldistortion in a cooling process after thermal treatment. Moreover, A1₂O₃ satisfies all other conditions as above-mentioned in relation to acap layer. Furthermore, an A12O3 cap layer can be formed more easilyespecially by sputtering and has an advantage to be easily etched byacid or alkaline etchant. Therefore the same effect is expected when A1₂O₃ is used for a cap layer in this invention.

As has been described above, in the present invention, after aheat-resistant cap layer of AlN in which a p-type dopant is added or Al₂O₃ is deposited, heat treatment for p-type layers is carried out. Thus,the heat treatment can be performed at higher temperatures, and hydrogencan be removed while curbing evaporation of the constituent element suchas nitrogen. Thus, the activation ratio of Mg increases and alow-resistance, high-quality p-type nitride compound layer having goodsurface flatness can be formed without adding excessive Mg. Therefore, ahigh-performance, short-wavelength light emitting element such as asemiconductor laser can be realized.

In the above description, Mg is mentioned as an example of the acceptorimpurity. The same advantage, however, can be obtained by carrying outheat treatment with a cap layer including such a doping material as tofunction as an acceptor of a nitride-based Group III-V compoundsemiconductor and to take in hydrogen in a doping layer at the sametime.

(Embodiment 3)

FIG. 11 is a graph showing variations in resistance value of p-typelayers in relation to heat treatment temperatures, in the case whereGaN, to which Mg is added in an amount of 5×10¹⁹ cm⁻³, is subjected toheat treatment with use of the acceptor-doped AlN cap layer of thepresent invention, the GaN is subjected to heat treatment with use of anAlN cap layer doped with no acceptor, and the GaN is subjected to heattreatment without use of the cap layer.

When the cap layer is not used, the resistance value decreases in a heattreatment temperature range of 400° C. or above. The resistance valuefurther decreases as the rise in treatment temperature. In a range of700° C. to 900° C. the lowest resistance value is obtained. However, theresistance increases in a heat treatment temperature range of over 900°C. When the AlN-undoped cap layer is used, the resistance valuedecreases in a simple manner as the treatment temperature increases. Ina range of 800° C. or above, the resistance value reaches a saturatedlevel. On the other hand, in the method of the present invention, theresistance decreases remarkably as the treatment temperature rises evenin a range of 700° C. or above. In a range of 800° C. or above, theresistance reducing effect does not greatly vary, but the resistancedecreases in a simple manner up to a treatment temperature range of1000° C. or above.

When the cap layer is not used, the hydrogen removing effect is improvedwith temperatures in a treatment temperature range of 700° C. or above.However, the degree of evaporation of the constituent element such asnitrogen increases simultaneously and the crystalline defectdeteriorates further. It is considered that consequently the resistancevalue reducing effect is prevented, and the resistance value rises,although the treatment temperature is increased, in a temperature rangeof 900° C. or above. Dissociation of the constituent element from thesurface can be prevented by using the AlN cap layer. In the case of notadding an acceptor, however, the hydrogen permeability is low and theresistance value reaches a saturated level. By contrast, in the methodof the present invention, there is no increase in defect in a range of700° C. or above, at which dissociation of the constituent element hasbeen unignorable in the prior art, and hydrogen can be eliminatedeffectively. Therefore, a crystal of a lower resistance than in theprior art is obtained. This effect is conspicuous in a treatmenttemperature range of 800° C. or above.

The effect of improvement in hydrogen permeability of AlN by addition ofan acceptor is obtained by addition of the acceptor in an amount of 10¹⁷cm⁻³. It is not desirable to add the acceptor in an amount of 10²⁰ cm⁻³or more, since such an amount of acceptor degrades the quality of thelayer.

FIG. 12 shows the relationship between the Mg concentration and theresistance value in a GaN layer which is subjected to heat treatment at800° C. for 30 minutes in an Ar atmosphere. In a conventional methodusing no cap layer, a decrease in resistance value does not occur unlessMg is added in an amount of 10¹⁹ cm⁻³ which results in cloudiness. Bycontrast, according to the method of the present invention wherein theacceptor-doped cap layer is used, the electrical activation ratio of Mgcan be increased. Thus, the resistance value can be decreased at an Mgconcentration of 10¹⁹ cm⁻³ or less at which a flat film is obtainable.These advantages are obtained similarly with a mixed crystal of AlN,InN, etc.

FIG. 13 is a cross-sectional view of the device structure of asemiconductor laser fabricated by the method of the third embodiment ofthe present invention. On the C face of a sapphire substrate 70, thefollowing layers are successively formed: an AlN (10 nm) first bufferlayer 71, a GaN (1.0 μm) second buffer layer 72, an Si-doped n-typeAlGaN (1.0 μm) cladding layer 73, a GaN (0.5 μm) active layer 74, anMg-doped p-type AlGaN (1.0 μm) cladding layer 75, and an Mg-doped p-typeGaN (0.5 μm) contact layer 76. Although not shown, electrodes areprovided on a side surface of the n-type cladding layer 73 and a topsurface of the p-type contact layer 76.

Like the semiconductor laser of Embodiment 1, the semiconductor laser ofthe present embodiment can be fabricated by using the growing apparatus(FIG. 4) used in Embodiment 1. In the present embodiment, in order toincrease the activation ratio of p-type dopant Mg in the Mg-doped p-typeAlGaN cladding layer 75 and p-type GaN contact layer 76, an Mg-doped AlNcap layer 77 (10 to 1000 nm) is formed on the contact layer 76, as shownin FIG. 14. After the resultant is cooled to room temperature, heattreatment is carried out at 800 to 1200° C. in a hydrogen-free gas suchas nitrogen or in a vacuum. Thereafter, the AlN cap layer 77 is removedby an acid containing hydrochloric acid or phosphoric acid or an alkalietching liquid containing NaOH, KOH, etc. The heat treatment may becarried out in a vacuum, but it is effective to perform it in ahydrogen-free gas since evaporation of the constituent element can beprevented.

More specifically, AlGaN is grown by introducing, as material, NH₃ at1×10⁻³ mol/min., Ga(CH₃)₃ at 1×10⁻⁵ mol/min., and Al(CH₃)₃ at 1×10⁻⁶mol/min. In the case of GaN, the supply of Al is stopped. In the case ofAlN, the supply of Ga is stopped. The temperature of the substrate is1050° C., the pressure is 75 Torr, and the flow rate of the totalmaterial gas is 1 l/min. The doping material for the cladding layers 73and 75 is SiH₄ as n-type material and (C₅ H₅)₂ Mg as p-type material.The heat treatment after AlN cap layer 77 formation is performed at1000° C. for 30 minutes in Ar atmosphere in the growing apparatus asshown in FIG. 4. The cap layer 77 is etched away by hydrochloric acid at60° C. for 15 minutes.

In the thus fabricated semiconductor laser, the Mg activation rate ofthe p-type GaN-based compound semiconductor layers (AlGaN cladding layer73 and GaN contact layer 75) can be increased. Thus, without excessivelyadding Mg, a low-resistance, p-type GaN-based compound semiconductorlayer can be formed. Therefore, a high-quality, p-type GaN-basedcompound semiconductor layer can be grown and the performance and lifeof the short-wavelength semiconductor laser can be increased.

As a modification of the third embodiment of this invention, there isfabricated a semiconductor laser in which an A1₂ O₃ cap layer is formedin place of an Mg-doped AlN cap layer and other conditions are all thesame as the third embodiment. More specifically, an A1₂ O₃ cap layer isformed by use of sputtering, the resultant is cooled to room temperatureand subjected to heat treatment at 1000° C. for 30 minutes in Aratmosphere, and the A1₂ O₃ cap layer is etched away by hyrdochloric acidat 70° C. for 15 minutes. In this case as well, the Mg activation rateof the obtained p-type GaN-based compound semiconductor layers can beincreased. Without excessively adding Mg, a low resistance andhigh-quality p-type GaN-based compound semiconductor layer can be grown.

The present invention is not limited to the above embodiments. In thethird embodiment, GaN or AlGaN is used as material of a low-resistance,p-type compound semiconductor layer. However, the present invention isnot limited to such a GaN-based compound semiconductor, and anitride-based Group III-V compound semiconductor may be used. Thethickness of the cap layer, the doping amount of the acceptor, the heattreatment temperature after the formation of the cap layer, etc. may bemodified on an as-needed basis. Besides, the present invention isapplicable not only to the semiconductor laser but also to themanufacture of a light emitting diode. Furthermore, this invention isapplicable to the manufacture of various semiconductor devices havinglayers of p-type nitride-based Group III-V compounds. Othermodifications may be made to the invention without departing from thespirit of the invention.

As has been described above, according to the present invention, theacceptor-doped AlN or A1₂ O₃ cap layer is formed on the acceptor-dopednitride-based Group III-V compound semiconductor layer, and theresultant structure is subjected to heat treatment. Thus, the heattreatment temperature is raised and hydrogen can be removed whilecurbing evaporation of a constituent element such as nitrogen.Therefore, the electrical activation ratio of acceptor impurity in thenitride-based Group III-V compound semiconductor layer can be increasedand the high-quality p-type layer can be formed. This contributes torealization of a high-performance, short-wavelength semiconductor laser,etc.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, and representative devices shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

What is claimed is:
 1. A semiconductor device comprising:a buffersemiconductor layer made of Al_(1-s-t) Ga_(s) In_(t) N (0≦s≦1, 0≦t≦1,s+t≦1) and having a number of pinholes formed therein; a thermaldistortion reducing layer made of Al_(1-u-v) Ga_(u) In_(v) N (0≦u≦1,0≦v≦1, u+v≦1) formed on said buffer semiconductor layer and having adifferent chemical formula from that of said buffer semiconductor layer,a first cladding layer formed on said thermal distortion reducing layer;an active layer formed on said first cladding layer; and a secondcladding layer formed on said active layer.
 2. The semiconductor deviceaccording to claim 1, wherein, in said Al_(1-u-v) Ga_(u) In_(v) N(0≦u≦1, 0≦v≦1, u+v≦1) for said thermal distortion reducing layer , v isset to be not less than 0.1 and not more than 0.9.
 3. A semiconductordevice according to claim 1, wherein a film thickness of said thermaldistortion reducing layer is greater than that of said semiconductorlayer.
 4. The semiconductor device according to claim 1, furthercomprising a cap layer on said thermal distortion reducing layer toprevent evaporation of In included in said thermal distortion reducinglayer.
 5. The semiconductor device according to claim 4, wherein saidcap layer is made of Al_(1-x) Ga_(x) N (0≦x≦1) and is formed at 500° C.to 800° C.
 6. The semiconductor device according to claim 1, whereinsaid first cladding layer is made of Al_(1-x-y) Ga_(x) In_(y) N (0≦x≦1,0≦y≦1, x+y≦1).
 7. The semiconductor device according to claim 1, whereinsaid thermal distortion reducing layer has a thickness of 50 nm to 1000nm.
 8. A semiconductor device according to claim 1, further comprising asingle crystal substrate on which said semiconductor layer is formed. 9.A semiconductor laser comprising:a first layer; a second layer made ofAl_(1-u-v) Ga_(u) In_(v) N (0≦u≦1, 0≦v≦1, u+v≦1) formed on said firstlayer, a third layer formed on said second layer; an active layer formedon said third layer; and a fourth layer formed on said active layer,wherein said first layer is formed of Al_(1-s-t) Ga_(s) In_(t) N (0≦s≦1,0≦t≦1, s+t≦1)with an average film thickness of 3 nm to 10 nm such thatsaid first layer has a number of pinholes formed among said Al_(1-s-t)Ga_(s) In_(t) N (0≦s≦1, 0≦t≦1, s+t≦1).
 10. A secmiconductor laseraccording to claim 9, further comprising a single crystal substrate onwhich said first layer is formed.